Method for coating heat resistant alloys

ABSTRACT

The substrate of a heat resistant alloy is coated with an oxidation resistant coating metal while substantially avoiding the formation of an internally oxidized structure within the coating by embedding a heat resistant alloy article to be coated in a cementation pack comprising said coating metal (e.g. chromium) which has a lower propensity to oxidize than an oxidation resistant solute metal in the alloy (e.g. aluminum), and then subjecting the alloy article to pack cementation at an elevated temperature while maintaining the oxygen in the pack at a partial pressure below the oxygen threshhold level at which the alloy is subject to internal oxidation.

O. Umted States Patent 1191 1111 3,801,353

Brill-Edwards Apr. 2, 1974 METHOD FOR COATING HEAT 3,276,903 10/1966 Galmiche 117 1072 P RESISTANT ALLOYS 3,073,015 1/1963 Wachtell et a1. 117/l07.2 P

3,290,126 12/1966 Monson 117/107.2 P [75] Inventor: Harry W. Brill-Edwards,

Edgewater Primary Examiner-Alfred L. Leavitt [73] Assignee: Chromalloy American Corporation, Assistant Examiner-Michael W. Ball Orangeburg, N.Y, Attorney, Agent, or FirmSandoe, Hapgood & 22 Filed: Apr. 11, 1972 Calimafde 21 A 1. No.: 242 951 1 pp 57 ABSTRACT Related US. Application Data 1 Th I b t t f h t t t H t d th esusraeoa ea res1san aoy1s coae w1 [62] 3232 2? June 1970 an oxidation resistant coating metal while substantially avoiding the formation of an internally oxidized struc- [52] U S Cl 117/71 29/194 29/197 turewithin the coating by embedding a heat resistant 3 117/107 2 P 416/24 alloy article to be coated in a cementation pack com- [51] Int- CL" C23c 11/00 C23c 13/00 G23c 17/02 prising said coating metal (e.g. chromium) which has [58] Field of Search 117/107 416/241, a lower propensity to oxidize than an oxidation resis- 39/194 198 tant solute metal in the alloy (e.g. aluminum), and then subjecting the alloy article to pack cementation [56] References Cited at an elevated temperature while maintaining the oxygen in the pack at a partial pressure below the oxygen UNITED STATES PATENTS threshhold level at which the alloy is subject to inter- 3,694,255 9/1972 Brill-Edwards 117/1072 P n oxidation 3,257,227 6/1966 Sedig 117/1072 P 3,065,108 11/1962 Sedig et al..... 117/107.2 P 9 Claims, 6 Drawing Figures METHOD FOR COATING HEAT RESISTANT ALLOYS This application is a division of application Ser. No. 43,082, filed June 3, 1970, now U.S. Pat. No. 3,694,255.

This invention relates to the diffusion coating of metals, such as heat resistant superalloys, and, in particular, to a process for producing an adherent oxidation and sulfidation resistant coating comprising chromium and aluminum on superalloy substrates while avoiding internal oxidation within the coating. The invention is particularly applicable to high nickel alloys for high temperature applications in, for example, jet and other thermal engines or gas turbines.

Metallurgical developments in recent years have indicated the necessity of high nickel and/or high cobalt alloys (sometimes now referred to as superalloys) having desirable physical properties for various high temperature uses, such as, for example, the manufacture of rotor blades and stator vanes for high temperature gas turbines where operation without failure is desired of the part, such as during prolonged exposure to temperatures well above 1,500F and even substantially above the temperature range at which failure or diminution of the strength characteristics may be expected of even high temperature austenitic or nickel or chromium steels.

Although the nickel or cobalt superalloys (and even those based on iron) may exhibit physical properties within a desirable range for a variety of uses, particularly when subjected in use to extremely high temperatures, the combination of such properties as oxidation resistance and/or erosion or sulfidation resistance at the surface of such alloys, the resistance to thermal shock and the strength characteristics may be less than desired for prolonged severe use. As operating temper atures were raised higher and higher, increased amounts of hardeners, such as aluminum and/or titanium, were added to certain nickel-base alloys in order to assure stiffness at the high temperature levels. As the hardeners increased in amount, it was not unusual to decrease the amount of chromium in such alloys in order to increase high temperature strength. A case in point is a nickel-base alloy referred to in the trade by the designation 8-1900 which comprises by weight about 0.1 percent carbon, 8 percent chromium, 10 percent cobalt, 6 percent molybdenum, 1 percent titanium, 6 percent aluminum, 0.015 percent baron, 0.1 percent zirconium, 4 percent tantalum and the balance essentially nickel. A disadvantage of such alloys is that with decreased amounts of chromium, the resistance to oxidation and sulfidation is generally adversely affected. Usually, 18 percent chromium or better is required for sulfidation resistance.

A chromium-free alloy which has shown particular promise as a jet engine or gas turbine component and which exhibits excellent stress-rupture properties at temperatures as high as 2,200F and above is an alloy comprising about 18 percent molybdenum, about 8 percent aluminum and the balance essentially nickel. However, this alloy does not have the desirable oxidation and/or sulfidation resistance and, therefore, its application in the gas turbine field has been severely limited due to the lack of a coating. At about 8 percent aluminum, the alloy contains gamma prime precipitate (Ni Al). The absence of chromium stabilizes the fine precipitate of the gamma prime to relatively high temperatures, for example, to above 2,200F as compared to 1,950F for such gamma prime strengthened alloys as the alloy bearing the designation 8-1900 referred to hereinabove. The alloy also contains a small amount of carbon, e.g. 0.03 percent carbon by weight, which appears as molybdenum carbide precipitated throughout the matrix. The superior mechanical properties are attributed to its lack of chromium, to its high molybdenum content, and to the relative absence of grain boundary carbides. However, this alloy must be used in the coated condition in order to withstand oxidation and sulfidation attack in gas turbine environments.

Initial attempts to produce a duplex chromium- /aluminum coating on this alloy in conventional cementation packs were not successful due to internal oxidation occurring within the coating during the initial chromizing step. Broadly speaking, the procedure usually employed'was to embed the metal article to be coated in a dry powder pack, including an inert mineral filler (e.g. powdered alumina), a source of chr0- mium to be diffusion coated and a source of a vaporizable halogen material. A typical pack is one containing by weight about 25 percent chromium, about 1/4 percent of a halide energizer (e.g. ammonium iodide) and the balance alumina. As embedded in such a pack contained within a metal container or retort, the seams of which are sealed by a fusible material, such as low melting silicate glass, to prevent excessive excape of the diffusing material during heating and inhibit introduction of air in the pack during the thermal cycle, the metal article is heated in a known manner to a diffusion temperature and held at temperature for a number of hours to cause diffusion coating of the chromium into the substrate of the article. Thereafter, the chromized article is aluminized, using another pack cementation procedure.

As stated above, the initial chromizing step applied to the high molybdenum-aluminum nickel-base superalloy resulted in localized internal oxidation within the chromium affected zones. This oxidation manifested itself in the form of angular inclusions of aluminum oxide particles within the chromium rich solid solution region of the coating, the size of the inclusions increasing with increasing coating temperature. Generally, the internal oxidation concentrated near the surface of the coating in which the aluminum concentration had been reduced by dissolution of the chromium from about 8 percent aluminum to a level below 3 percent by weight. The internal oxidation was not observed by penetrate through the coating diffusion zones into the base metal where the aluminum level approached 8 percent by weight. The low chromium, nickel-base superalloys were observed to exhibit the same phenomenon, such as alloys of the type referred to herein as 8-1900.

The aluminum oxide inclusions have an adverse effeet on the properties of the alloy in that the presence of inclusions tends to reduce the resistance of the coating to impact and, moreover, the aluminum oxide/coating matrix interfaces introduce regions susceptible to rapid corrosion attack.

While attempts were made to overcome the foregoing problem, none, as far as it is known, has been successful.

It is thus an object of the invention to provide a method of coating metal articles with oxidation and sulfidation resistant coatings while inhibiting internal oxidation within the coating.

Another object is to provide a pack cementation method for coating the substrate of an article made of a heat resistant superalloy in which the heat resistant alloy contains a solute metal (e.g. aluminum) in an amount having a higher propensity to oxidize than the primary coating metal (e.g. chromium) in the cementation pack and the solvent metal of the alloy, while substantially avoiding internal oxidation within the coating.

A still further object is to provide a multistep pack cementation process for producing a multiple oxidation and sulfidation corrosion resistant coating on superalloys substantially free from internal oxidation near the interface of the coating.

The invention also provides a process for coating a molybdenum-aluminum alloy containing major amounts of nickel.

These and other objects will more clearly appear when taken in conjunction with the following disclosure and the accompanying drawings, wherein:

FIGS. 1 and 2 are reproductions of photomicrographs taken at 400 times magnification showing the internal oxidation which occurs within a coating produced by a method outside of the invention;

FIG. 3 is a reproduction of a photomicrograph taken at 400 times magnification showing the same alloy composition with no internal oxidation within the coating produced in accordance with the invention;

FIG. 4 is illustrative of a photomicrograph taken at 400 times magnification showing the extent to which internal oxidation occurs when the pack cementation process of the invention is carried out in a retort without an adequate seal;

FIG. 5 is a reproduction of a photomicrograph taken at 400 times magnification showing a coating produced using a pack composition outside the invention depicting nickel aluminide build-up and the attendant entrapment of alumina particles from the pack; and

FIG. 6 is a reproduction of a photomicrograph taken at 400 times magnification showing the various phases making up a duplex coating produced in accordance with the invention while avoiding the formation of inclusions of aluminum oxide in the coating.

In its broad aspects, the invention is directed to a method for coating by pack cementation the substrate of an article formed of a heat resistant alloy in which the heat resistant alloy contains a solute metal in amounts having a relatively higher propensity to oxidize than the coating metal in the cementation pack and the solvent metal of the base alloy such that normally an internally oxidized structure is produced within the coating during pack cementation comprising an oxide dispersion of the oxidizable solute metal due to the presence of oxygen in the pack. The improvement resides in embedding the alloy article containing the solute metal in a cementation pack comprising a coating metal having a lower propensity to oxidize than the solute metal, and then carrying out the cementation process at an elevated coating temperature while maintaining the oxygen in the pack at a partial pressure below the oxygen threshhold level at which the alloy is subject to internal oxidation.

The solute metal in the alloy to be coated includes those metals the oxides of which have a negative free energy of formation of at least about 115,000 calories per gram atom of oxygen at about 25C, and generally at least about 125,000 calories, e.g. 133,000 calories or higher. Examples of such solute metals normally present in superalloys are aluminum, titanium, and the like. The invention is particularly applicable to heat resistant alloys of the nickel-base type containing aluminum and/or titanium hardeners.

The invention is further applicable to pack cementation systems in which chromium is the initial coating metal and in which the solute metal in the alloy is aluminum. In coating an alloy containing aluminum as the solute metal, a preferred method of maintaining the oxygen in the pack to below the threshhold level is' to add to the pack an efiective amount of a getter whose propensity to oxidize is at least equal to that of the solute metal in the alloy to be coated. A getter found particularly satisfactory is aluminum powder in effective amounts ranging up to about 1.25 percent, the amount of aluminum being advantageously less than that amount which interferes with the difi'usion of chromium into the substrate and which tends to cause the attendant entrapment of pack material in the coating. For example, if excess aluminum is employed, nickel aluminide may tend to form on the substrate which then inhibits diffusion of chromium therein. A range of aluminum content by weight in the pack found particularly advantageous is about 0.25 percent to 0.75 percent.

As stated hereinabove, the invention is applicable to the coating of a broad range of alloy compositions, such as those containing by weight up to about 30 percent of a metal from the group consisting of Cr, Mo and W, the total of these metals not exceeding about 40 percent, with the Cr content preferably not exceeding about 10 or 15 percent, up to about 10 percent by weight of a metal from the group consisting of Cb and Ta; up to about 1% C; up to about 10 percent by weight of a metal from the group consisting of Ti and Al, the total amount of these metals not exceeding about 12 percent; up to about 2% Mn; up to about 2% Si; up to about 0.1% B; up to about 1% Zr; and the balance at least about percent nickel. As will be appreciated, the iron group metals Fe and/or Co may be substituted for at least part of the nickel. For example, the alloy may contain up to 20 percent or more of cobalt.

The invention is particularly applicable to the coating of nickel-molybdenum-aluminum alloys containing by weight up to about 30 percent molybdenum (more advantageously about 10 to 25 percent), about 0.5 to 10 percent aluminum (more advantageously about 4 to 10 percent), up to about 0.1 percent carbon (e.g. 0.03 percent), and the balance essentially nickel. A specific example of the alloy (known commercially as NX- 188) is one containing approximately 17.5 percent molybdenum, approximately 7.75 percent aluminum, approximately 0.03 percent carbon, and the balance essentially nickel.

Examples of other alloys capable of being coated in accordance with the invention are as follows:

" 'ofiE'MIoAL o6 masmmfi wmm 1 E RoENT Alloy designation C Cr Ni Co Mo W Cb Fe Ti Al 13 Ar Others 0.0 T2). 'IRW-OA 0.13 0.1 Hal... 7.5 2.0 5.8 0.6 1.0 5.4 0.02 0.13 0.311%.

0. e. Mar-M-246 0.15 0. Ba]. 10. 0 2. 5 10. 0 1.5 5. 5 0. 015 0. 05 1.5 T8. Mar-M-432 0. 16 16.4 13211.. 10. 7 3. 0 1. 8 4. 4 2. 0 0. 013 0. 03 2.2 Ta. 15-1900 0.10 8. 0 1381.--. 10. 0 6. 0 1.0 0. 0 0. 015 0.10 40 Ta.

0.08 15.0 Bal 18.5 5.2 3.5 4.3 0.03 0.08 18. 0 Bal. 18. 5 4. 0 2. 9 2. 0 0. 006 0. 05 0.13 15.0 Bel"-.. 8.5 1.8 2.6 0.0 0.1 3.5 3.4 0.011 0.11 19 Ta. 0.21 12. 7 Hal... 8. 0 2.0 3. 9 4. 2 3. 2 0. 02 0.1 3 9 Ta. 0.12 12. 6 Ba] 4. 2- 2. 0 0. 8 6.1 0.012 0. 10

Th goxyge n threshhold level should desirably not exceed 4 parts per million of the gas in the pack.

A chromizing pack which is particularly advantageous in carrying out the invention is one containing by weight about 5 to percent chromium, about 0.25 to 0.75 percent aluminum, about 3 to 5 percent nickel and the balance essentially an inert diluent, such as a refractory oxide, for example, alumina. The pack has mixed with it an effective amount of a halide energizer, e.g. one-quarter percent. The function of nickel in the above pack is to maintain the aluminum potential below a level that causes nickel aluminide to form on the alloy, while, at the same time, not suppressing the chromium transfer rate.

Examples of other halide energizers are ammonium iodide, ammonium bromide, ammonium bifiuoride, and the like. By employing the pact of the invention in an adequately sealed retort, the internal oxidation of the chromized layer is inhibited. While it is possible through very special care to reduce internal oxidation by means of an appropriate seal, the use of a getter assures maintaining the oxygen level in the pack to below the oxygen threshhold level above which internal oxidation occurs and thus substantially avoid oxidation.

Once an adequate first coating is produced substantially free from internal oxidation, additional coating of other metals can be applied,,e.g. aluminum.

Thus, the invention is also applicable to multistep pack cementation processes. Broadly, a typical multistep process for coating heat resistant alloys may comprise, providing a first cementation pack containing a first coating metal (e.g. chromium) have a lower propensity to oxidize than the solute metal in the alloy (e.g. aluminum) to be coated, embedding the heat resistant alloy in the pack, carrying out a first cementation process at an elevated coating temperature while maintaining the oxygen in the pack at a partial pressure below the oxygen threshhold level at which the alloy is subject to internal oxidation, whereby the alloy is coated with the first coating metal while substantially avoiding internal oxidation, embedding the coated alloy in a second cementation pack containing at least one other coating metal (e.g. aluminum) and carrying out the second cementation at an elevated tempera ture.

A cementation pack which may be employed in the second coating step for transferring, for example, aluminum as the second coating material, comprises about 5 to percent of a buffering metal (e.g. chromium), about 1.25 to 20 percent aluminum and the balance essentially an inert diluent containing a small but effective amount of a halide energizer, the amount of aluminum at the higher range being correlated to the higher range of the buffering metal (e.g. chromium), with the lower range of aluminum being correlated to the lower range of the buffering metal. The buffering metal aids in controlling the transfer and deposition of the aluminum. Examples of other buffering metals are nickel, iron and cobalt.

In carrying out the primary chromizing step, it is preferred that the pack be prepared by first pre-reacting it at an elevated temperature, for example 1,850F to 2,200F for from 1 to 20 hours. The pre-reacted pack is then reenergized by mixing it with a small but effective amount of ammonium halide, e.g., one-quarter percent ammonium bromide, and the article to be coated then pack chromized at about 1,900F to 2,200F for times up to about 50 hours, eg 30 hours at 2,000F for the alloy NX-l88.

A typical chromizing pack (the first coat) is one prepared from about 15 percent by weight of 20 40 mesh Cr, 4 percent by weight of minus 200 mesh Ni, 3/4 or 1 percent by weight of aluminum powder of size of about minus 325 mesh, one-quarter percent NH Br and the balance l4 28 mesh A1 0 A typical aluminizing pack (the second coat) is 22 percent by weight of 20 40 mesh Cr, 8 percent by weight of aluminum powder of size of about minus 325 mesh, onequarter percent of NH FHF and the balance Al O of -14 28 mesh. The coating with the latter pack is carried out at 1,700F for 20 hours. Broadly speaking, the temperal ture may range from about 1,600F to 2,000F for up to about 50 hours, e.g. 10 to 30 hours.

When the two-step coating procedure is employed to coat the alloy comprising 17.5% Mo-7.75% Al-bal. nickel alloy using the foregoing packs, the final coating on the metal substrate comprises an outer layer of nickel aluminide phase with dispersed alpha chromium particles Cr-20% Mo). Moving further inward from the aute'f'i'ayer, the composition of the white precipitated phaseschanges from chromium-rich behind the aluminized zone to molybdenum-rich near the substrate.

The chromized layer is not sufficient by itself to provide the necessary protection against hot corrosion. For example, a chromized layer (5 mil thick) produced in accordance with the invention von an alloy substrate comprising about 17.5% Mo, about 7.75% Al and the balance essentially Ni exhibited good resistance to sulfidation. However, oxidation exposure at 2,200F caused sufficient vaporization of the oxidized chromium surface thereby reducing the life over which the coating afforded sulfidation resistance. On the other hand, when the chromized substrate was aluminized to form nickel aluminide, the resulting duplex coating exhibited excellent oxidation and sulfidation resistance.

As illustrate of the various embodiments of the invention, the following examples are given.

Example 1 In chromizing an element (such as a vane or blade) made of an alloy comprising approximately 17.5 percent molybdenum, approximately 7.75 percent aluminum, approximately 0.03 percent carbon and the balance essentially nickel, a chromizing pack is first prepared by mixing by weight percent of 40 mesh chromium powder, 4 percent minus 200 mesh nickel powder, 1 percent minus 325 mesh aluminum powder, 1/4% NI-I Br and the balance essentially 14 28 mesh A1 0 The mixed powders are prereacted at 2,100F for 20 hours and then re-energized with 1/4% N11,,Br.

The pre-reacted re-energized pack is placed in a retort and the element embedded in the pack. The retort is sealed with a low melting silicate glass composition and the retort then heated in a muffle furnace to a temperature of 2,l00F and held at temperature for about 30 hours. The retort is thereafter cooled to room temperature and the chromized element (referred to as Test No. 1) is metallographically examined for internal oxidation.

For comparison purposes, two tests (No. 2A and No. 3A) were carried out on elements of the same composition in which the chromizing pack used comprised by weight percent chromium powder, 1/4% NH Br and the balance A1 0; without the presence of a getter. In Test 2A, the retort was sealed with the low melting silicate glass mentioned hereinabove while in Test 3A, no seal was employed. In both cases, the element was chromized at 2,100F for hours.

With regard to Test Nos. 2A and 3A, metallographic examination revealed internal oxidation in the coating as will be apparent from FIG. 1 (Test No. 2A) and FIG. 2 (Test No. 3A) taken at 400 times magnification. It will be noted that the amount of internal oxidation is greater where no seal is employed in the retort. However, in both cases (FIGS. 1 and 2), the internal oxidation is substantial.

As regards Test No. 1 in which the glass sealed pack contained chromium, nickel and aluminum, the presence of aluminum as a getter prevented internal oxida tion as will be apparent from FIG. 3 which is free ofinternal oxidation.

A test conducted (Test No. 4A) in which the chromium-nickel-aluminum pack of the invention was employed (15% Cr, 4% Ni, 1% Aland the balance A1 0 but in which no seal was used in the retort, resulted? a substantial amount of internal oxidation in the coating as will be apparent from FIG. 4 taken at 400 times magnification.

The foregoing tests of Example 1 confirm that unless the partial pressure of oxygen in the pack is maintained low enough, internal oxidation will occur during chromizing. This is also true even if inadequate quantities of the getter are present or if the seal is inadequate.

A test, Test 5A, was conducted on the Ni-Mo-Al alloy using a chromizing pack containing 15 percent chromium, 4 percent nickel with 3 percent aluminum as the getter under a good retort seal. The metal substrate was processed at a temperature of 2,100F for 30 hours and the metallographic structure shown in FIG. 5 was obtained. As will be observed from the photomicrograph, a nickel aluminide layer was formed having entrapped therein alumina particles from the pack. Because nickel aluminide was deposited on the substrate,

the chromium transfer was suppressed. This type of coating does not provide adequate hot corrosion protection of the alloy at elevated temperatures ranging up to about 2,200F.

Example 2 An alloy referred to by the designation B-1900 may similarly be chromized in accordance with the invention while avoiding internal oxidation. This alloy contains nominally 0.1 percent carbon, 8 percent chromium, 10 percent cobalt, 6 percent molybdenum, 1 percent titanium, 6 percent aluminum, 0.015 percent boron, 0.1 percent zirconium, 4 percent tantalum and the balance essentially nickel. As in Example 1, an element of the alloy is embedded in a pre-reacted pack contained in a sealed retort comprising by weight about 10 percent of --20 40 mesh chromium powder, about 3 percent minus 200 mesh nickel powder, about 0.5 percent minus 325 mesh aluminum, about NH,l and the balance A1 0 of about -14 28 mesh. As in Example 1, the mixed powders are pre-reacted at 2,100F for 20 hours and then re-energized with of NH I. The pre-reacted re-energized pack is placed in' a retort and the element of the alloy embedded in the pack, the retort being then sealed. The assembly is heated in a furnace to a temperature of about 1,900F for 30 hours and thereafter cooled to room temperature to produce a chromized element substantially free from internal oxidation.

Example 3 An element of an alloy referred to by the designation TRW-6A and comprising about 0.13 percent carbon, 6.1 percent chromium, 7.5 percent cobalt, 2 percent molybdenum, 5.8 percent tungsten, 0.5 percent columbium, 1 percent titanium, 5.4 percent aluminum, 0.02 percent boron, 0.13 percent zirconium, 9 percent tantalum, 0.4 percent hafnium, 0.14 percent rhenium and the balance essentially nickel is chromized similarly as in Example 1, except that the pre-reacted pack contains about 5 percent chromium, 3 percent nickel, 0.35 percent aluminum, NH FHF and the balance alumina. The assembly of the retort containing the pack with the embedded element therein is then sealed and heated to about 1,925F and held at temperature for about 30 hours and thereafter cooled to room temperature to provide a chromized element substantially free from internal oxidation. As stated hereinabove, once the first metal coating has been deposited on any of the alloys disclosed herein substantially fee from internal oxidation, a second protective coating can be easily applied. Thus, the alloy of nickel-molybdenum-aluminum coated in Example 1 (Test No. 1) in accordance with the invention may be aluminized by using a pre-reacted pack composition comprising by weight 22 percent of 20 40 mesh chromium powder as a buffering metal, 8 percent of minus 325 mesh aluminum powder, 14% ammonium bifluoride (Nl-I FHF) and the balance an inert refractory oxide, e.g. -14 28 mesh A1 0 The pack with the embedded element is sealed in a retort and then aluminized at a temperature of about 1,700F for about 20 hours. A duplex coating is produced comprising a surface layer of nickel aluminide (NiAl) containing a dispersion of alpha chromium (Mo) particles. The subsurface coating comprises a nickel-chromium solid solution containing aluminum in solid solution, the

amount of aluminum remaining substantially constant at about 7.5 to 8 percent at the aluminide interface to the nominal 7.5 or 8 percent at the base alloy interface. Molybdenum rich phases are formed in the chromium solid solution adjacent the substrate. The foregoing metallographic structure will be apparent by referring to FIG. 6 taken at 400 times magnification.

Referring to FIG. 6, it will be noted that the surface layer is enriched in nickel aluminide, the layer below and adjacent it comprising a layer of chromium rich solid solution. A layer of molybdenum rich phases is interposed between the chromium rich solid solution and the metal substrate comprised of the nickelmolybdenum-aluminum alloy containing approximately 17.5 percent molybdenum, approximately 7.75 percent aluminum and the balance essentially nickel. The foregoing layers are metallurgically bonded to each other and to the substrate.

By producing the foregoing coating using the invention, internal oxidation is substantially avoided and a high quality oxidation and sulfidation resistant coating obtained, particularly on nickel-molybdenum.- aluminum alloys containing up to about 30 percent molybdenum, about 0.5 percent to 10 percent aluminum and the balance essentially nickel.

The temperature which may be employed to aluminize the chromized alloy may range from about 1,600F to 2,000F for up to about 50 hours, e.g. 10 to 30 hours.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

What is claimed is:

l. A multi-step pack cementatlon process utilizing ih tlie pact; said process inhibiting the roraaiafi'ar" said internally oxidized structure, said process comprises.

dispersion of said osidizable solute metal due t o'tli presence of oxygen in the pack, said process inhibiting the formation of said internally oxidized structure, said process comprises,

1. embedding the alloy article in said first cementation pack, said pack comprising chromium, an inert diluent, a small but effective amount of a halide energizer, and an effective amount of a getter whose propensity to oxidize is at least equal to that of the solute metal in the alloy, the getter being one whose negative free energy of formation of the oxide at about 25C is at least about 1 15,000 calories per gram atom of oxygen,

the amount of the getter ranging up to about L25 percent by weight of the pack and being less than that amount which interferes with the diffusion of chromium into the substrate and which tends to cause entrapment of pack material in the coat- 2. carrying out the cementation in a sealed container at an elevated chromizing temperature,

whereby a diffusion-bonded chromium layer is produced on the substrate of said article substantially free from internal oxidation,

3. embedding said coated alloy in a second cementation pack containing at least one coating metal, 4. and carrying out said second cementation at an elevated coating temperature to produce a final metal coating different from said chromium layer.

2. The method of claim 1, wherein the heat resistant alloy contains by weight up to about 30 percent of a metal from the group consisting of Cr, Mo and W, the total of these metals not exceeding about 40 percent; the chromium content of the alloy ranging up to about 15 percent; up to about 10 percent by weight of a metal from the group consisting of Cb and Ta; up to about 1 percent Co; up to about 10 percent by weight of a metal from the group consisting of Ti and Al, the total amount of these metals not exceeding about 12 percent; up to about 20 percent cobalt; up to about 2% Mn; up to about 2% Si; up to about 0.1% B; up to about 1% Zr; and the balance at least about 50 percent nickel.

3. A multi-step pack cementation process for producing an oxidation and sulfidation corrosion resistant coating comprising a top coat of nickel-aluminum on an initial coating which is produced by chromizing by pack cementation in a sealed container the substrate of an article formed of a heat resistant alloy containing by weight up to about 25 percent molybdenum, about 0.5 to 10 percent aluminum as an oxidizable gamma prime forming solute metal, and the balance essentially nickel, the oxidizable solute metal having a relatively higher propensity to oxidize than the chromium coating metal in the cementation pack such that an internally oxidized structure is produced in said coating during paek cementation comprising an o xidedispersion of aluminum due to the presence of oxygeriin the pack confined in the sealed container, said process greatly inhibiting the formation of an internally oxidized structure, said process comprises,

1. embedding said alloy article in said cementation pack comprising chromium, an inert diluent, a small but effective amount of a halide energizer, and an effective amount of a getter whose propensity to oxidize is at least equal to that of the solute metal in the alloy, the getter being one whose negative free energy of formation of the oxide at about 25C is at least about 115,000 calories per gram atom'of oxygen, the amount of the getter ranging up to about 1.25

percent by weight of the pack and being less than the amount which interferes with the diffusion of chromium in the substrate and which tends to cause entrapment of pack material in the coating,

2. carrying out the cementation process in a sealed container at an elevated chromizing temperature,

I whereby a diffusion-bonded chromium layer is obtained on the substrate of said article substantially free from internal oxidation,

3. embedding said coated alloy in a second cementation pack containing aluminum and a buffering metal for controlling the transfer and deposition of said aluminum onto the coated alloy,

4. and carrying out said second cementation at an elevated coating temperature, whereby to produce a final coating of nickelchromium-aluminum.

4. The method of claim 3, wherein the buffering metal is chromium.

5. The method of claim 3, wherein the heat resistant alloy contains about to 25 percent molybdenum and about 4 to 10 percent aluminum.

6. The method of claim 5, wherein the alloy contains approximately 17.5 percent molybdenum, approximately 7.75 percent aluminum and approximately 0.03 percent carbon.

7. The method of claim 3, wherein the first cementation pack contains by weight about 5 to 15 percent chromium, about 0.25 to 0.75 percent aluminum, and about 3 to 5 percent nickel.

8. The method of claim 3, wherein the second cementation pack contains about 5 to 40 percent chromium, 1.25 to 20 percent aluminum, such that the higher range of aluminum is employed with the higher range of chromium and such that the lower range of aluminum is employed with the lower range of chromium, and the balance essentially an inert diluent containing a small but effective amount of a halide energizcr.

9. The method of claim 3, wherein the final coating contains substantial amounts of nickel aluminidc. 

2. carrying out the cementation process in a sealed container at an elevated chromizing temperature, whereby a diffusion-bonded chromium layer is obtained on the substrate of said article substantially free from internal oxidation,
 2. carrying out the cementation in a sealed container at an elevated chromizing temperature, whereby a diffusion-bonded chroMium layer is produced on the substrate of said article substantially free from internal oxidation,
 2. The method of claim 1, wherein the heat resistant alloy contains by weight up to about 30 percent of a metal from the group consisting of Cr, Mo and W, the total of these metals not exceeding about 40 percent; the chromium content of the alloy ranging up to about 15 percent; up to about 10 percent by weight of a metal from the group consisting of Cb and Ta; up to about 1 percent Co; up to about 10 percent by weight of a metal from the group consisting of Ti and Al, the total amount of these metals not exceeding about 12 percent; up to about 20 percent cobalt; up to about 2% Mn; up to about 2% Si; up to about 0.1% B; up to about 1% Zr; and the balance at least about 50 percent nickel.
 3. A multi-step pack cementation process for producing an oxidation and sulfidation corrosion resistant coating comprising a top coat of nickel-aluminum on an initial coating which is produced by chromizing by pack cementation in a sealed container the substrate of an article formed of a heat resistant alloy containing by weight up to about 25 percent molybdenum, about 0.5 to 10 percent aluminum as an oxidizable gamma prime forming solute metal, and the balance essentially nickel, the oxidizable solute metal having a relatively higher propensity to oxidize than the chromium coating metal in the cementation pack such that an internally oxidized structure is produced in said coating during pack cementation comprising an oxide dispersion of aluminum due to the presnce of oxygen in the pack confined in the sealed container, said process greatly inhibiting the formation of an internally oxidized structure, said process comprises,
 3. embedding said coated alloy in a second cementation pack containing at least one coating metal,
 3. embedding said coated alloy in a second cementation pack containing aluminum and a buffering metal for controlling the transfer and deposition of said aluminum onto the coated alloy,
 4. and carrying out said second cementation at an elevated coating temperature, whereby to produce a final coating of nickel-chromium-aluminum.
 4. The method of claim 3, wherein the buffering metal is chromium.
 4. and carrying out said second cementation at an elevated coating temprerature to produce a final metal coating different from said chromium layer.
 5. The method of claim 3, wherein the heat resistant alloy contains about 10 to 25 percent molybdenum and about 4 to 10 percent aluminum.
 6. The method of claim 5, wherein the alloy contains approximately 17.5 percent molybdenum, approximately 7.75 percent aluminum and approximately 0.03 percent carbon.
 7. The method of claim 3, wherein the first cementation pack contains by weight about 5 to 15 percent chromium, about 0.25 to 0.75 percent aluminUm, and about 3 to 5 percent nickel.
 8. The method of claim 3, wherein the second cementation pack contains about 5 to 40 percent chromium, 1.25 to 20 percent aluminum, such that the higher range of aluminum is employed with the higher range of chromium and such that the lower range of aluminum is employed with the lower range of chromium, and the balance essentially an inert diluent containing a small but effective amount of a halide energizer.
 9. The method of claim 3, wherein the final coating contains substantial amounts of nickel aluminide. 